The present invention relates to a platform and system for conducting offshore hydrocarbon recovery operations. More particularly, the present invention relates to a platform structure and system for allowing the use of a jack-up rig in deeper water.
Jack-up rigs provide a derrick and associated equipment for drilling, completing or working over a well. This equipment is mounted to a combined hull.backslash.deck which is capable of floating these facilities to site. A plurality of retractable legs are provided which renders the jack-up rig conveniently portable. Once floated into position for conventional operations, the legs are jacked-down until they engage the seafloor. Further jacking transfers the load from the buoyant hull to the legs, then lifts the hull/deck out of the water and above the splash zone to produce a stable, bottom founded offshore platform for conducting well operations.
A consideration of this design is that to best take advantage of the mobile nature of the facilities provided on the jack-up rig, the rig is removed after drilling is complete and does not remain deployed at the development during the production phase except, possibly, for temporary drilling and workover operations. The considerable investment in drilling, completion and workover equipment is best utilized by redeploying the jack-up rig to another location as soon as these operations are complete. Thus, surface completions for production are not accommodated on the jack-up rig itself. A small structure called a "well jacket" can be used with the jack-up rig to provide the benefits of a surface completion with the convenience of a jack-up rig. However, well jackets and jack-up rig combinations are limited to shallow water deployment. Further, practical limitations on the length of the retractable legs more directly restrict the depth in which jack-up rigs can be traditionally deployed.
The requirements of deeper water depths have most often been answered by the continued use of traditional bottom founded platform structures. Topside facilities provide convenient well access for production operations. However, such structures must dedicate a significant amount of their structural strength to supporting drilling facilities that are only required for a relatively short period of time in the life of the overall operations from the platform in recovering oil and gas from a reservoir. Further, the structure must be able to withstand the maximum design environmental conditions, the design hurricane criteria, with these drilling facilities in place.
Of course, recovery operations lead to depletion of the hydrocarbon reservoir and, in time, the platform loses its usefulness at a site. Nevertheless, the well jacket that forms the tower supporting the deck of the platform may be structural sound and capable of an extended useful life. However, salvage operations are difficult and another constraint of traditional well jackets is that they are design specific for a given water depth. This tends to substantially limit redeployment opportunities.
Certain designs have been proposed for "piggyback" deployment of a jack-up rig onto a subsea structure, yet these designs have carried forward many of the limitations of each structure producing a result that, although it increases water depth for the jack-up rig, otherwise remains the sum of the limitations of its constituent parts.
More recently a new platform concept has been proposed combining the benefits of jack-up rigs and traditional bottom founded platform structures, without carrying their drawbacks into the combination. Thus, the "Hyjack" platform has been proposed which combines a small surface tower sufficient to support production operations with a substantial jacket base which supports the surface tower and temporarily supports a jack-up rig for drilling operations. Following drilling, the jack-up rig is moved off and the small surface tower supports production operations. This is described in greater detail in U.S. patent application Ser. No. 08/129,820, filed Sep. 30, 1993, by Dale M. Gallaher et al for an Offshore Platform Structure and System. Further features that facilitate salvage and redeployment, particularly in combination with the foregoing platform concept, are described more fully in U.S. patent application Ser. No. 08/129,829, filed Sep. 30, 1993, by George E. Sgouros et al for a Reusable Offshore Platform Jacket. The full disclosure of each of these patent applications are hereby incorporated by reference and made a part hereof.
As platforms are used in progressively deeper water, their dynamic response may become a greater design consideration as the traditional bottom-founded platforms become relatively less rigid in response to wind, wave and currents. However, dynamic response becomes of a central concern for compliant towers where flexibility is a key design precept. Compliant towers are designed to "give" in a controlled manner in response to dynamic environmental loads rather than to nearly rigidly resist those forces.
A basic requirement in controlling this response is to produce a structure having harmonic frequencies or natural periods that avoid those encountered in nature. The total mass at the top to the jacket base is one of the controlling variables in defining the natural periods of the structure. Adaptation of the hyjack platform concept to compliant towers represents a unique challenge is because one platform must accommodate such widely different design states based upon the presence or absence of the jack-up rig at the time in question.
Thus, there continues to be a need in some circumstances for economically accommodating and even enhancing the benefits of surface completions and the convenience and economies of jack-up rig operations in deeper water, particularly for compliant tower applications in which the dynamic response of the offshore platform structure in a function of the mass of the total offshore platform and must accommodate operations both with the jack-up rig in place and with it removed.